Trouble diagnosis device for exhaust gas purification system and trouble diagnosis method for exhaust gas purification system

ABSTRACT

There are provided a trouble diagnosis device and a trouble diagnosis method for an exhaust gas purification system in which the presence or absence of a trouble of the exhaust gas purification system such as abnormality of a control system for the exhaust gas purification system, catalyst deterioration or the like can be determined with high precision, and the reliability of the exhaust gas purification system can be enhanced. 
     The device includes upstream-side NO x  flow rate calculating means, downstream-side NO x  flow rate calculating means, reduction condition determining means for determining whether at least one condition for performing normal reduction of NO x  is satisfied, upstream-side NO x  flow amount calculating means for integrating the upstream-side NO x  flow amount to calculate the amount of NO x  passing through the upstream side of the catalyst within a predetermined time when the condition concerned is determined to be satisfied, downstream-side NO x  amount calculating means for integrating the downstream-side NO x  flow amount to calculate the amount of NO x  passing through the downstream side of the catalyst within a predetermined time when the condition concerned is determined to be satisfied, and trouble determining means for comparing the upstream-side NO x  amount and the downstream-side NO x  amount to determine whether the exhaust gas purification system operates normally.

TECHNICAL FIELD

The present invention relates to a trouble diagnosis device for anexhaust gas purification system and a trouble diagnosis device for anexhaust gas purification system. Particularly, the present inventionrelates to trouble diagnosis device and method for an exhaust gaspurification system that can perform trouble determination by comparingan NO_(x) amount at the upstream side of NO_(x) catalyst and an NO_(x)amount at the downstream side of the NO_(x) catalyst.

BACKGROUND ART

Particular matter (hereinafter referred to as PM), NO_(x) (NO or NO₂),etc. which may induce a risk of affecting the environment are containedin exhaust gas discharged from an internal combustion engine such as adiesel engine or the like. An exhaust gas purification system usingNO_(x) catalyst disposed in an exhaust gas passage is known as anexhaust gas purification system used to purify NO_(x) out of the abovematerials.

An exhaust gas purification system using NO_(x) storage catalyst and anSCR (Selective Catalytic Reduction) system using selective reductioncatalyst are known as such an exhaust gas purification system.Accordingly to the exhaust gas purification system using the NO_(x)storage catalyst, NO_(x) in exhaust gas is absorbed under the state thatthe air-fuel ratio of the exhaust gas is under a lean state, and whenthe air-fuel ratio of the exhaust gas is changed to a rich state, NO_(x)is subjected to reductive reaction with hydro carbon (HC) and carbonoxide (CO) in the exhaust gas while NO_(x) is discharged, therebypurifying the exhaust gas. Furthermore, the SCR system uses catalyst forselectively reducing NO_(x) in exhaust gas and supplies reducing agentmainly containing urea or HC into the exhaust gas so that NO_(x) issubjected to reductive reaction with the catalyst, thereby purifying theexhaust gas.

In these exhaust gas purification systems, there is a case whereoriginally-expected reduction of NO_(x) does not occur due to someabnormality of the system itself, deterioration of the catalyst, defectof the reducing agent or the like. However, even when the purificationefficiency of NO_(x) is lowered, no trouble occurs on operation.Therefore, the operation is continued without taking any action.Accordingly, NO_(x) is discharged into atmospheric air, which affectsthe environment.

Therefore, a technique of self-diagnosing reduction of the purificationcoefficient of NO_(x) in the exhaust gas purification system has beenproposed. For example, a catalyst deterioration diagnosis device for aninternal combustion engine which diagnoses reduction of NO_(x)conversion performance due to deterioration of catalyst has beenproposed (see Patent Document 1). More specifically, there is discloseda catalyst deterioration diagnosis device for an internal combustionengine in which NO_(x) sensors are provided at the upstream side anddownstream side of the catalyst, the output of each NO_(x) sensor ismeasured, the ratio in NO_(x) concentration between the upstream anddownstream sides of the catalyst, that is, the NO_(x) conversion rate iscalculated, and a reference value of the NO_(x) concentration ratio isset on the basis of the rotational number of the internal combustionengine and the basic fuel injection amount. The NO_(x) concentrationratio is compared with the reference value, and the reduction of theNO_(x) conversion rate is determined on the basis of the comparisonresult.

Furthermore, there has been disclosed a method of diagnosing thedeterioration of the NO_(x) catalyst by estimating the amount of NO_(x)captured by the NO_(x) catalyst (the NO_(x) amount at the upstream sideof the NO_(x) catalyst) from the operation state of the internalcombustion engine, etc. and using an integration value of the NO_(x)concentration detected by NO_(x) sensor at the downstream side withoutusing the NO_(x) concentration itself detected by the NO_(x) sensor ofthe internal combustion engine (for example, see Patent Document 2).

Patent Document 1: JP-A-7-54641 (Scope of Claim for Patent, FIG. 5)

Patent Document 2: JP-A-2005-54604 (Claims 7, 12, paragraphs [0047] to[0048])

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

The purification efficiency of NO_(x) may be reduced irrespective of thepresence or absence of the catalyst deterioration in accordance withconditions such as the temperature of the NO_(x) catalyst, the NO_(x)concentration at the upstream side of the NO_(x) catalyst, the flow rateof exhaust gas, etc. Furthermore, the purification efficiency of NO_(x)may be reduced due to abnormality of the control system for the exhaustgas purification system, defect of reducing agent, deterioration ofoxidation catalyst when the oxidation catalyst is disposed at theupstream side of the NO_(x) catalyst or the like.

However, the catalyst deterioration diagnosis disclosed in the PatentDocument 1 and the Patent document 2 is the deterioration diagnosiswhich pays no attention to the condition as described above. Therefore,there is a risk that the deterioration is determined to progress morebitterly as compared with the actual catalyst deterioration degree, sothat the reliability of the diagnosis result may be lowered.

Therefore, the inventors of this invention have made an earnest effort,and found that the problem as described above can be solved byperforming trouble determination on the basis of integrated NO_(x)amount only when a predetermined condition is satisfied in the casewhere the NO_(x) amounts at the upstream and downstream sides of NO_(x)catalyst are compared with each other to perform the troubledetermination of an exhaust gas purification system, therebyimplementing this invention. That is, an object of the present inventionis to provide a trouble diagnosis device for an exhaust gas purificationsystem and a trouble diagnosis method for an exhaust gas purificationsystem in which the presence or absence of a trouble of the exhaust gaspurification system such as abnormality of a control system for theexhaust gas purification system, catalyst deterioration or the like canbe determined with high precision, and the reliability of the exhaustgas purification system can be enhanced.

Means of Solving the Problem

According to the present invention, there is provided a troublediagnosis device for an exhaust gas purification system for passingexhaust gas discharged from an internal combustion engine through NO_(x)catalyst to reduce NO_(x) contained in the exhaust gas that comprises:upstream-side NO_(x) flow rate calculating means for calculating theupstream-side NO_(x) flow amount per unit time at the upstream side ofthe NO_(x) catalyst; downstream-side NO_(x) flow rate calculating meansfor calculating a downstream-side NO_(x) flow amount per unit time atthe downstream side of the NO_(x) catalyst; reduction conditiondetermining means for determining whether at least one condition forperforming normal reduction of NO_(x) is satisfied; upstream-side NO_(x)flow amount calculating means for integrating the upstream-side NO_(x)flow amount to calculate the amount of upstream-side NO_(x) passingthrough the upstream side of the NO_(x) catalyst within a predeterminedtime when the condition concerned is determined to be satisfied;downstream-side NO_(x) amount calculating means for integrating thedownstream-side NO_(x) flow amount to calculate the amount ofdownstream-side NO_(x) passing through the downstream side of the NO_(x)catalyst within a predetermined time when the condition concerned isdetermined to be satisfied; and trouble determining means for comparingthe upstream-side NO_(x) amount and the downstream-side NO_(x) amount todetermine whether the exhaust gas purification system operates normally,whereby the above problem can be solved.

In the construction of the trouble diagnosis device for the exhaust gaspurification system according to the present invention, it is preferablethat the upstream-side NO_(x) amount calculating means and thedownstream-side NO_(x) amount calculating means store the integrationvalues which have been obtained till now when the integration of theNO_(x) flow amount is interrupted, and resume the integration from thestored integration values when the condition concerned is satisfiedagain.

In the construction of the trouble diagnosis device for the exhaust gaspurification system according to the present invention, it is preferredthat the upstream-side NO_(x) amount calculating means and thedownstream-side NO_(x) amount calculating means store the integrationvalues which have been obtained till now when the integration of theNO_(x) is interrupted, and reset the integration values when thecondition concerned is not satisfied again within a predetermined time.

In the construction of the trouble diagnosis device for the exhaust gaspurification system according to the present invention, it is preferredthat the trouble determining means finishes the integration of theupstream-side NO_(x) flow amount and the downstream-side NO_(x) flowamount when the upstream-side NO_(x) amount reaches a predeterminedvalue, and compares the upstream-side NO_(x) amount and thedownstream-side NO_(x) amount.

Furthermore, in the construction of the trouble diagnosis device for theexhaust gas purification system according to the present invention, itis preferable that the trouble determining means compares the ratiobetween the upstream-side NO_(x) amount and the downstream-side NO_(x)amount with a threshold value which is determined in accordance with acondition under which reduction of NO_(x) is normally performed.

Still furthermore, in the construction of the trouble diagnosis devicefor the exhaust gas purification system according to the presentinvention, it is preferable that the upstream-side NO_(x) flow ratecalculating means performs the calculation on the basis of an NO_(x)concentration discharged from the internal combustion engine which iscalculated from an operation state of the internal combustion engine.

Furthermore, according to another aspect of the present invention, atrouble diagnosis method for an exhaust gas purification system fordiagnosing the presence or absence of a trouble of an exhaust gaspurification system in which exhaust gas discharged from an internalcombustion engine is passed through NO_(x) catalyst to reduce NO_(x)contained in the exhaust gas, comprises: calculating an upstream-sideNO_(x) flow amount and a downstream-side NO_(x) flow amount per unittime at the upstream and downstream sides of the NO_(x) catalyst;determining whether at least one condition under which the reduction ofNO_(x) is normally performed is satisfied or not; integrating theupstream-side NO_(x) flow amount and the downstream-side NO_(x) flowamount when the condition concerned is satisfied, and calculating anupstream-side NO_(x) amount and the downstream-side NO_(x) amountpassing through the upstream side and the downstream side of the NO_(x)catalyst within a predetermined time; and determining comparing theupstream-side NO_(x) amount with the downstream-side NO_(x) amount todetermine whether the exhaust gas purification system operates normally.

Effect of the Invention

According to the trouble diagnosis device for the exhaust gaspurification system according to the present invention, the NO_(x)amounts at the upstream side and the downstream side of the NO_(x)catalyst which are integrated only when the condition for performing thenormal reductive reaction of NO_(x) is satisfied are compared with eachother, whereby a reduction efficiency when the condition concerned isnot satisfied can be excluded from materials for performing thedetermination. Accordingly, there can be provided the trouble diagnosisdevice for the exhaust gas purification system that can determine thepresence or absence of a trouble of the exhaust gas purification systemsuch as abnormality of a control system for the exhaust gas purificationsystem, deterioration of NO_(x) catalyst, reduction of the quality ofreducing agent or the like.

Furthermore, in the trouble diagnosis device for the exhaust gaspurification system according to the present invention, when theintegration of the NO_(x) flow amount is interrupted and then resumed,the integration is resumed from the integration value at theinterruption time, whereby the trouble diagnosis can be efficientlyperformed without starting the trouble diagnosis from the beginning evenwhen the operation state of the internal combustion engine is unstable.

Still furthermore, in the trouble diagnosis device for the exhaust gaspurification system according to the present invention, the integrationvalue is reset when the time period for which the integration of theNO_(x) flow amount is interrupted is continued for a predetermined timeor more, whereby reliability of a determination result can be enhanced.

Still furthermore, in the trouble diagnosis device for the exhaust gaspurification system according to the present invention, the integrationis finished when he upstream-side NO_(x) amount reaches a predeterminedvalue, and the upstream-side NO_(x) amount and the downstream-sideNO_(x) amount are compared with each other, whereby a time required forthe trouble diagnosis can be prevented from being excessivelylengthened, so that the efficient trouble diagnosis can be performed.

Still furthermore, in the trouble diagnosis device for the exhaust gaspurification system according to the present invention, the troubledetermination is carried out by comparing the reduction efficiency ofNO_(x) with a predetermined threshold value, whereby the determinationcan be properly performed in accordance with the operation state of theinternal combustion engine or the like.

Still furthermore, in the trouble diagnosis method for the exhaust gaspurification system according to the present invention, theupstream-side NO_(x) flow amount is calculated, not on the basis of adetection value obtained by an NO_(x) sensor, but on the basis of anestimation value estimated from the operation state of the internalcombustion engine, whereby the number of NO_(x) sensors is reduced andthe increase of the cost can be suppressed.

Still furthermore, according to the trouble diagnosis method for theexhaust gas purification system according to the present invention, thereduction efficiency under the state that the condition for performingthe normal reductive reaction of NO_(x) is not satisfied is excludedfrom materials for performing the determination. Accordingly, thepresence or absence of a trouble of the exhaust gas purification systemsuch as abnormality of the control system for the exhaust gaspurification system, deterioration of the NO_(x) catalyst, reduction ofthe quality of reducing agent or the like can be determined with highprecision.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] is a diagram showing an example of the construction of anexhaust gas purification system according to an embodiment of thepresent invention.

[FIG. 2] is a block diagram showing an example of the construction of atrouble diagnosis device for the exhaust gas purification system.

[FIG. 3] is timing chart showing how to integrate an NO flow amount.

[FIG. 4] is a flowchart (part 1) showing an example of a troublediagnosis method for the exhaust gas purification system according tothe embodiment of the present invention.

[FIG. 5] is a flowchart (part 2) showing an example of the troublediagnosis method for the exhaust gas purification system according tothe embodiment of the present invention.

[FIG. 6] is a flowchart (part 3) showing an example of the troublediagnosis method for the exhaust gas purification system according tothe embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments concerning a trouble diagnosis device for an exhaust gaspurification system and a trouble diagnosis method for an exhaust gaspurification system according to the present invention will be describedspecifically with reference to the drawings. However, these embodimentsare examples of the present invention. Accordingly, the presentinvention is not limited to these embodiments, and thus theseembodiments may be arbitrarily altered within the scope of the presentinvention.

In the figures, the same reference numerals represent the same members,and the description is fitly omitted.

1. Trouble Diagnosis Device for Exhaust Gas Purification System

(1) Whole Construction of Exhaust Gas Purification System

First, an example of the construction of an exhaust gas purificationsystem having a trouble diagnosis device for an exhaust gas purificationsystem according to an embodiment (hereinafter referred to as “system”in some cases) will described with reference to FIG. 1.

An exhaust gas purification system 10 shown in FIG. 1 uses urea watersolution as reducing agent, and passes exhaust gas through NO_(x)catalyst 13 together with the reducing agent to selectively reduceNO_(x). The exhaust gas purification system 10 has a NO_(x) catalyst 13which is disposed at some point of an exhaust gas passage 11 connectedto an internal combustion engine and selectively reduces NO_(x)contained in exhaust gas, and a reducing agent supply device 20containing a reducing agent injection valve 31 for injecting reducingagent into the exhaust gas passage 11 at the upstream side of the NO_(x)catalyst 13. Temperature sensors 15 and 16 are disposed at the upstreamside and the downstream side of the NO_(x) catalyst 13 of the exhaustgas passage 11 respectively, and also an NO_(x) sensor 17 asdownstream-side NO_(x) concentration calculating means is disposed atthe downstream side of the NO_(x) catalyst 13. The constructions of theNO_(x) catalyst 13, the temperature sensors 15, 16 and the NO_(x) sensor17 are not limited to specific ones, and well-known members may be used.

However, in the exhaust gas purification system of this embodiment, atleast the NO_(x) sensor 17 has a trouble diagnosis function for thesensor itself, and outputs error information to CAN (Controller AreaNetwork) 65 described later when an abnormal state is detected.

The reducing agent supply device 20 has an injection module 30containing a reducing agent injection valve 31, a storage tank 50 inwhich reducing agent is stocked, a pump module 40 containing a pump 41for pressure-feeding the reducing agent in the storage tank 50 to thereducing agent injection valve 31, and a control unit (hereinafterreferred to as “DCU: Dosing Control unit”) 60 for controlling theinjection module 30 and the pump module 40 to control the injectionamount of the reducing agent to be injected from the reducing agentinjection valve 31. The storage tank 50 and the pump module 40 areconnected to each other through a first supply passage 57, the pumpmodule 40 and the injection module 30 are connected to each otherthrough a second supply passage 58, and the injection module 30 and thestorage tank 50 are connected to each other through a circulationpassage 59.

In the example of the exhaust gas purification system 10 shown in FIG.1, DCU 60 is connected to CAN 65. CAN 65 is connected to a control unit(hereinafter referred to as “ECU: Engine Control Unit” in some cases) 70for controlling the operation state of the internal combustion engine.Not only information concerning the operation state of the internalcombustion engine such as a fuel injection amount, an injection timing,a rotational number, etc. are written in CAN 65, but also information ofall sensors, etc. provided to the exhaust gas purification system 10 arewritten in CAN 65. In CAN 65, it can be determined whether an inputsignal value is within a standard range or not of CAN 65. DCU 60connected to CAN 65 can read information on CAN 65, and also outputinformation onto CAN 65.

In this embodiment, ECU 70 and DCU 60 comprise separate control units,and can communicate information with each other through CAN 65. However,ECU 70 and DCU 60 may be constructed as a single control unit.

The storage tank 50 is provided with a temperature sensor 51 fordetecting the temperature of reducing agent in the tank, a level sensor55 for detecting the residual amount of reducing agent and a qualitysensor 53 for detecting quality such as viscosity, concentration, etc.of reducing agent. Values detected by these sensors are output assignals and written onto CAN 65. Well-know parts may be properly used asthese sensors.

Urea water solution, hydro carbon (RC) is mainly used as the reducingagent to be stocked, and the exhaust gas purification system of thisembodiment is constructed to use urea water solution.

The pump module 40 has a pump 41, a pressure sensor 43 as pressuredetecting means for detecting the pressure in the second supply passage58 at the downstream of the pump 41 (hereinafter referred to as“pressure of reducing agent” in some cases), a temperature sensor 45 fordetecting the temperature of reducing agent to be pressure-fed, aforeign material collecting filter 47 disposed at some position of thesecond supply passage 58 at the downstream side of the pump 41, and apressure control valve 49 for returning a part of the reducing agentfrom the downstream side of the pump 41 to the upstream side of the pump41 to reduce the pressure when the pressure of the reducing agent at thedownstream side of the pump 41 exceeds a predetermined value.

The pump 41 comprises an electrically-operated pump, for example, and itis driven on the basis of a signal transmitted from DCU 60. Well-knownsensors may be properly used as the pressure sensor 43 and thetemperature sensor 45. Values detected by these sensors are output assignals, and written onto CAN 65. Furthermore, a well-known check valveor the like may be used as the pressure control valve 49.

The injection module 30 has a pressure chamber 33 in which reducingagent pressure-fed from the pump module 40 side is stocked, a reducingagent injection valve 31 connected to the pressure chamber 33, anorifice 35 disposed at some position of a passage extending from thepressure chamber 33 and intercommunicating with the circulation passage59, and a temperature sensor 37 disposed just before the orifice 35.

The reducing agent injection valve 31 comprises an ON-OFF valve forcontrolling ON-OFF of valve opening through duty control, for example.Furthermore, in the pressure chamber 33, the reducing agent pressure-fedfrom the pump module 40 is stocked under a predetermined pressure, andthe reducing agent is injected into the exhaust gas passage 11 when thereducing agent injection valve 31 is opened on the basis of a controlsignal transmitted from DCU 60. The orifice 35 is disposed in thepassage at the downstream side of the pressure chamber 33, so that theinternal pressure of the pressure chamber 33, the second supply passage58 at the upstream side of the orifice 35 is not lowered easily, andthus the output of the pump module 40 can be suppressed to a low value.As not shown, a valve for performing circulation control of reducingagent may be provided at some position of the circulation passage 59 inplace of arranging the orifice 35.

Furthermore, the circulation passage 59 disposed between the injectionmodule 30 and the storage tank 50 is provided so that reducing agentother than reducing agent injected from the reducing agent injectionvalve 31 of the injection module 30 is made to reflow into the storagetank 50 out of reducing agent pressure-fed from the pump module 40 inorder to prevent the reducing agent from being affected by exhaust gasheat or the like and exposed to high temperature.

DCU 60 performs the operation control of the reducing agent injectionvalve 31 on the basis of various information existing on CAN 65 so thata proper amount of reducing agent is injected into the exhaust gaspassage 11. Furthermore, DCU 60 in the embodiment of the presentinvention has a function as a trouble diagnosis device for the exhaustgas purification system 10 (hereinafter referred to as “troublediagnosis device”).

DCU 60 mainly comprises a microcomputer having a well-knownconstruction, and in FIG. 1, constructions represented by functionalblocks are shown with respect to parts concerning the operation controlof the reducing agent injection valve 31, the driving control of thepump 41 and the trouble diagnosis of the exhaust gas purification system10.

That is, DCU 60 in the embodiment of the present invention comprises, asmain constituent elements, a CAN information taking and generating unit(represented as “CAN information take-out and generation” in FIG. 1), atrouble diagnosis unit of the exhaust gas purification system(represented by “system trouble diagnosis” in FIG. 1), a pump drivingcontrol unit (represented as “pump driving control” in FIG. 1), areducing agent injection valve operation controller (represented as “Udvoperation control” in FIG. 1), etc. Specifically, these parts areimplemented by executing programs through the microcomputer (not shown).

The CAN information taking and generating unit reads informationconcerning the driving state of the internal combustion engine outputfrom ECU 70, sensor information output from the NO_(x) sensor 17 andinformation existing on CAN 65, and outputs these information onto therespective parts.

Furthermore, the pump driving controller continually reads informationconcerning the pressure of reducing agent in the second supply passage58 which is output from the CAN information taking and generating unit,and executes feedback control on the pump 41 on the basis of thepressure information, so that the pressure of the reducing agent in thesecond supply passage 58 and the pressure chamber 33 is kept to asubstantially fixed state. For example, in the case where the pump 41 isan electrically-operated pump, when the output pressure value is lowerthan a target value, the pump 41 is controlled so that the duty ratio ofthe electrically-operated pump is increased to increase the pressure.Conversely, when the output pressure value exceeds the target value, thepump 41 is controlled so that the duty ratio of theelectrically-operated pump is reduced to lower the pressure.

The reducing agent injection valve operation controller reads theinformation concerning the reducing agent in the storage tank 50, theinformation concerning the exhaust gas temperature, the NO_(x) catalysttemperature and the NO_(x) concentration at the downstream side of theNO_(x) catalyst, the information concerning the operation state of theinternal combustion engine, etc. which are output from the CANinformation taking and generating unit, generates a control signal forinjecting from the reducing agent injection valve 31 reducing agentwhose amount is required to reduce NO_(x) contained in exhaust gas, andoutputs the control signal to a reducing agent injection valve operationdevice (represented by “Udv operation device” in FIG. 1) 67 foroperating the reducing agent injection valve 31.

Purification of exhaust gas by the exhaust gas purification system 10constructed as shown in FIG. 1 is carried out as follows.

Under the operation of the internal combustion engine, the reducingagent in the storage tank 50 is pumped up by the pump 41, andpressure-fed to the injection module 30 side. At this time, thedetection value of the pressure sensor 45 at the downstream side of thepump 41 provided to the pump module 40 is fed back. When the detectionvalue is less than a predetermined value, the output of the pump 41 isincreased. When the pressure value exceeds a predetermined value, thepressure is reduced by the pressure control valve 49. Accordingly, thepressure of the reducing agent pressure-fed to the injection module 30side is controlled to be kept to a substantially fixed value.

Furthermore, the reducing agent pressure-fed from the pump module 40 tothe injection module 30 flows into the pressure chamber 33 of thereducing agent and it is kept to substantially fixed pressure, wherebythe reducing agent is injected into the exhaust gas passage 11 at alltimes when the reducing agent injection valve 31 is opened. Furthermore,the reducing agent reflows through the circulation passage 59 into thestorage tank 50. Therefore, the reducing agent which is not injectedinto the exhaust gas passage 11 is stocked in the pressure chamber 33,and thus it is prevented from being exposed to high temperature withexhaust gas heat.

Under the state that the reducing agent is stocked in the pressurechamber 33 under a substantially fixed pressure value, DCU 60 determinesthe amount of reducing agent to be injected on the basis of informationsuch as the operation state and exhaust gas temperature of the internalcombustion engine, the temperature of the NO_(x) catalyst 13 and theamount of NO_(x) which is passed through the NO_(x) catalyst 13 withoutbeing reduced and measured at the downstream side of the NO_(x) catalyst13, etc., generates the control signal corresponding to the determinedreducing agent amount and outputs the control signal to the reducingagent injection valve operation device (not shown). The duty control ofthe reducing agent injection valve 31 is performed by the reducing agentinjection valve operation device, and a proper amount of reducing agentis injected into the exhaust gas passage 11. The reducing agent injectedinto the exhaust gas passage 11 flows into the NO_(x) catalyst 13 whilemixed in the exhaust gas, and used for the reductive reaction of NO_(x)contained in the exhaust gas, whereby the purification of the exhaustgas is performed.

(2) Trouble Diagnosis Device

Here, DCU 60 of the embodiment of the present invention is provided witha trouble diagnosis unit of the exhaust gas purification system 10. Thetrouble diagnosis unit of the exhaust gas purification system 10compares the upstream-side NO_(x) amount and downstream-side NO_(x)amount of the NO_(x) catalyst which pass within a predetermined timeunder the state that a predetermined condition is satisfied, anddiagnoses whether the exhaust gas purification system operates normally.

As shown in FIG. 2, the trouble diagnosis unit of the exhaust gaspurification system has upstream-side NO_(x) concentration calculatingmeans for detecting the NO_(x) concentration at the upstream side of theNO_(x) catalyst (represented as “upstream-side NO_(x) concentrationcalculation” in FIG. 2), upstream-side NO_(x) flow rate calculatingmeans for calculating the NO_(x) flow amount at the upstream side of theNO_(x) catalyst per unit time (represented as “upstream-side NO_(x) flowrate calculation” in FIG. 2), upstream-side NO_(x) amount calculatingmeans for calculating the amount of NO_(x) passing through the upstreamside of the NO_(x) catalyst within a predetermined time (represented as“upstream-side NO_(x) amount calculation” in FIG. 2), downstream-sideNO_(x) flow rate calculating means for calculating the NO_(x) flow rateat the downstream side of the NO_(x) catalyst (represented as“downstream-side NO_(x) flow rate calculation” in FIG. 2), anddownstream-side NO_(x) amount calculating means for calculating theamount of NO_(x) passing through the downstream side of the NO_(x)catalyst within a predetermined time (represented as “downstream-sideNO_(x) amount calculation” in FIG. 2).

The trouble diagnosis unit contains exhaust gas mass flow ratecalculating means for calculating the mass flow rate of exhaust gas(represented as “exhaust gas mass flow rate calculation” in FIG. 2),catalyst temperature calculating means for calculating the temperatureof the NO_(x) catalyst from detection values of the temperature sensorsat the upstream side and downstream side of the NO_(x) catalyst(represented as “catalyst temperature calculation” in FIG. 2), reductioncondition determining means for determining whether at least onecondition for performing reduction of NO_(x) is satisfied or not(represented as “reduction condition determination” in FIG. 2), andtrouble determining means for determining the presence or absence of atrouble of the exhaust gas purification system by comparing theupstream-side NO_(x) amount and the downstream-side NO_(x) amount(represented as “trouble determination” in FIG. 2).

The exhaust gas mass flow rate calculating means reads informationconcerning the operation state of the internal combustion engine whichis output from the CAN information taking and generating unit, andcalculates the mass flow rate of the exhaust gas discharged from theinternal combustion engine.

Furthermore, as in the case of the exhaust gas flow rate calculatingmeans, the upstream-side NO_(x) concentration calculating means readsinformation concerning the operation state of the internal combustionengine which is output from the CAN information taking and generatingunit, and calculates the concentration of NO_(x) discharged from theinternal combustion engine.

A fuel injection amount, a rotational number, the status of an exhaustcirculation device (EGR: Exhaust Gas Recirculation), an exhaustcirculation amount, an air suction amount, a cooling water temperature,etc. are used as the information concerning the operation state of theinternal combustion engine which is used to calculate the exhaust gasmass flow rate and calculate the concentration of NO_(x) discharged fromthe internal combustion engine and exists on CAN. The calculation of theexhaust gas mass flow rate and the NO_(x) concentration on the basis ofthese information can be performed by a well-known method.

In DCU 60 of this embodiment, the NO_(x) concentration at the upstreamside of the NO_(x) catalyst is determined by the calculation. However,as in the case of the NO_(x) concentration at the downstream side of theNO_(x) catalyst, an NO_(x) sensor may be disposed at the upstream sideof the NO_(x) catalyst and the detection value of the NO_(x) sensorconcerned is used although the cost may rise up.

The upstream-side NO_(x) flow rate calculating means calculates theNO_(x) flow amount at the upstream side of the NO_(x) catalyst per unittime on the basis of the upstream-side NO_(x) concentration calculatedby the upstream-side NO_(x) concentration calculating means describedabove and the exhaust gas mass flow rate. The upstream-side NO_(x)amount calculating means integrates the NO_(x) flow rate in a time zonesatisfying a predetermined condition out of the NO_(x) flow ratecalculated by the upstream-side NO_(x) flow rate calculating means,thereby integrating the NO_(x) amount passing through the upstream sideof the NO_(x) catalyst within a predetermined time.

Furthermore, the downstream-side NO_(x) flow rate calculating meanscalculates the NO_(x) flow amount at the downstream side of the NO_(x)catalyst per unit time on the basis of the NO_(x) concentration detectedby the NO_(x) sensor disposed at the downstream side of the NO_(x)catalyst and the exhaust gas mass flow rate, which are output from theCAN information taking and generating unit. The downstream-side NO_(x)amount calculating means integrates the NO_(x) flow rate in a time zonesatisfying a predetermined condition out of the NO_(x) flow ratecalculated by the downstream-side NO_(x) flow rate calculating means,thereby calculating the NO_(x) amount passing through thedownstream-side of the NO_(x) catalyst within a predetermined time.

In DCU 60 of this embodiment, the upstream-side NO_(x) amountcalculating means and the downstream-side NO_(x) amount calculatingmeans integrate the NO_(x) flow rate only when it is determined inreduction condition determining means that the system satisfies variousconditions under which the reduction of NO_(x) is normally performed.The temperature of the NO_(x) catalyst, the concentration of exhaustedNO_(x), the flow rate of the exhaust gas, etc. are used as theconditions for normally performing the reduction of NO_(x). It is animportant element to the activation state of the catalyst whether thetemperature of the NO_(x) catalyst is within a predetermined range ornot.

Whether the exhausted NO_(x) concentration and the flow rate of theexhaust gas are within predetermined ranges is an important element towhether the flow rate of NO_(x) flowing into the NO_(x) catalyst iswithin a processing capacity range of the catalyst or not. That is, whenthe NO_(x) flow rate is integrated although these conditions are notsatisfied, the system is diagnosed to be broken down in spite of asituation that the reduction efficiently is merely low, and thus thereliability of the diagnosis result is lowered. Therefore, theintegration is performed only when the above conditions are satisfied.

The reduction condition determining means determines whether the exhaustgas purification system satisfies the various conditions for normallyperforming the reduction of NO_(x), and outputs signals to theupstream-side NO_(x) amount calculating means and the downstream-sideNO_(x) amount calculating means. These conditions contain thetemperature of the NO_(x) catalyst calculated by the catalysttemperature calculating means, the NO_(x) concentration at the upstreamside of the NO_(x) catalyst calculated by the upstream-side NO_(x)concentration calculating means, the exhaust gas mass flow ratecalculated by the exhaust gas flow rate calculating means, etc. Acondition range for normally performing the reduction of NO_(x) in theNO_(x) catalyst is defined in advance, and when these conditions arewithin the prescribed range, signals are output so that theupstream-side NO_(x) amount calculating means and the downstream-sideNO_(x) calculating means integrate the NO_(x) flow rate.

The catalyst temperature calculating means estimates the temperature ofthe NO_(x) catalyst by using map or the like on the basis of thetemperature information detected by the temperature sensors at theupstream side and downstream side of the NO_(x) catalyst output from theCAN information taking and generating unit. Here, the estimatedtemperature information of the NO_(x) catalyst is used as one of theconditions for normally performing the reduction of NO_(x).

Furthermore, RAM (Random Access Memory) is connected to the troublediagnosis unit of DCU 60 of this embodiment, and the integration valuesof the NO_(x) amounts which are calculated by the upstream-side NO_(x)amount calculating means and the downstream-side NO_(x) amountcalculating means are stored on a case-by-case basis.

The trouble determining means reads the upstream-side NO_(x) amount andthe downstream-side NO_(x) amount stored in RAM, and determines theratio of the downstream-side NO_(x) amount to the upstream-side NO_(x)amount. In addition, the trouble determining means compares thereduction efficiency of NO_(x) represented by this ratio with apredetermined threshold value to determine whether the system operatesnormally.

Furthermore, the trouble determining means is provided with an NO_(x)flow rate counter. When the condition for normally performing thereduction of NO_(x) is satisfied, the counter is subjected to additionprocessing. When the condition concerned is not satisfied, the counteris subjected to subtraction processing. The counter value is used todetermine whether the integration value of the NO_(x) flow rate isreliable to the extent that it can be used for the trouble diagnosis ofthe system.

Furthermore, in order to define the maximum time until the state thatthe system does not satisfy the above condition is continued and theintegration value of the NO_(x) flow rate is reset, the value of theNO_(x) flow rate counter is prevented from increasing to a predeterminedvalue MAX or more. The addition rate of the NO_(x) flow rate counter isincreased when the NO_(x) flow rate is high, and it is reduced when theNO_(x) flow rate is low. This is because the reliability of the troublediagnosis using the integration value of the NO_(x) flow rate is moreenhanced as the NO_(x) flow rate is higher, and the time taken until thecounter value is equal to zero by subjecting the counter to thesubtraction processing can be lengthened.

For descriptive purposes, the timing chart of FIG. 3 is illustratedwhile the addition rate is set to be fixed.

(3) Timing Chart

The upstream-side NO_(x) flow rate and the downstream-side NO_(x) flowrate are integrated in the trouble diagnosis device of this embodimentwhen the system satisfies various conditions for normally performing thereduction of NO_(x). Next, this will be described in detail withreference to the timing chart shown in FIG. 3.

First, when the state that the condition of normally performing thereduction of NO_(x) is satisfied (the state that Condition is set to astate indicating True) is set at a time point of t1, the NO_(x) flowrate counter is subjected to the addition processing (NO_(x) flow ratecounter Inc). The integration of the NO_(x) flow rate at the upstreamside of the NO_(x) catalyst is started by Pre-Integrator of theupstream-side NO_(x) amount calculating means at the time point of t1,and the integration is continued insofar as Condition is set to thestate of True.

Thereafter, since the condition for normally performing the reduction ofNO_(x) is not satisfied at a time point of t2 (Condition is set to astate indicating False), the NO_(x) flow rate counter is subjected tothe subtraction processing (NO_(x) flow rate counter Dec). Theintegration of the NO_(x) flow rate at the upstream side which iscontinually performed is interrupted at the time point of t2.Subsequently, the NO_(x) flow rate counter is also subjected to thesubtraction processing, and the integration value which is integrated byPre-Integrator is reset at a time point of t3 at which the counter valueof the NO_(x) flow rate counter is set to zero without exceeding aprescribed value START.

Subsequently, when Condition is set to True at a time point of t4 again,the addition processing of the NO_(x) flow rate counter is resumed, andalso the integration of the NO_(x) flow rate at the upstream side of theNO_(x) catalyst by Pre-Integrator is resumed. The state that Conditionis set to True is continued between t4 and t5, and thus the integrationof the NO_(x) flow rate at the upstream side of the NO_(x) catalyst isalso continued. Thereafter, at a time point of t5, Condition is set toFalse. Therefore, as in the case of the time point of t2, the NO_(x)flow rate counter is subjected to the subtraction processing, and alsothe integration of the NO_(x) flow rate at the upstream side isinterrupted.

Subsequently, at a time point of t6, Condition is set to True againbefore the NO_(x) flow rate counter is equal to zero. Therefore, theNO_(x) flow rate counter is subjected to the addition processing again,and also the integration of the upstream-side NO_(x) flow rate byPre-Integrator is resumed.

At this time, the NO_(x) flow rate counter exceeds the prescribed valueSTART at a time point of t7. Therefore, the integration value which isintegrated by Pre-Integrator is added to Main-Integrator, and also theswitching operation is carried out so that the subsequent integration isperformed by Main-Integrator. In the time chart of FIG. 3,Main-Integrator is set so that the previous integration value has beenalready stored therein. By selectively using Pre-Integrator andMain-Integrator, the storage of the integration value is carried out byMain-Integrator, and the integration at the initial stage is carried outby Pre-Integrator. Therefore, when the NO_(x) flow rate counter is resetat some midpoint at the initial stage of the integration, it can bereturned to zero at all times.

Subsequently, at a time point of t8, the NO_(x) flow rate counterexceeds the prescribed value MAX, and thus the NO_(x) flow rate counteris fixed to MAX insofar as Condition is set to True. As described above,the value of the NO_(x) flow rate counter is fixed to MAX at maximum inorder to prevent such a situation that the time taken until the statethat Condition is set to False is continued at some midpoint and thusthe NO_(x) flow rate counter is set to zero and thus reset and the timetaken until the integration is finally finished are excessivelylengthened, so that the determination precision is lowered and the timerequired for the trouble determination is lengthened.

Subsequently, the integration is interrupted under the state thatCondition is set to False, and the integration of Main-Integrator iscontinued under the state that Condition is set to True.

Subsequently, a timer 1 is actuated at a time point of t9 at which thevalue of the integrated upstream-side NO_(x) amount exceeds theprescribed value MIN. The integration of Main-Integrator is finished ata time point of t10 at which the period of the timer 1 is finished. Withrespect to the timing at which the integration of Main-Integrator isfinished, even before the timer 1 is finished, the integration isfinished when the period for which Condition indicates False iscontinued for a predetermined time after the value of the upstreamNO_(x) amount exceeds the prescribed value MIN.

As described above, The integration is finished after a predeterminedtime elapses from the time when the integration value of theupstream-side NO_(x) flow rate exceeds the prescribed value MIN becauseafter the prescribed value MIN corresponding to the lowest NO_(x)integration amount is secured when the trouble determination isperformed, the NO_(x) integration amount is further accumulated so thata required time is not remarkably lengthened, thereby enhancing thedetermination precision.

2. Trouble Diagnosis Method for Exhaust Gas Purification System

Next, a specific routine of the trouble diagnosis method for the exhaustgas purification system will be described with reference to theflowcharts of FIGS. 4 to 6. This routine may be executed at all times,or it may be executed by interrupting every fixed time.

First, after the routine is started, the mass flow rate Gf of theexhaust gas discharged from the internal combustion engine is calculatedin step S100, and then the NO_(x) concentration in the exhaust gasdischarged from the internal combustion engine, that is, the NO_(x)concentration Nu at the upstream side of the NO_(x) catalyst iscalculated in step S101. Subsequently, in step S102, the NO_(x) flowrate Nfu per unit time at the upstream side of the NO_(x) catalyst iscalculated on the basis of the mass flow rate Cf of the exhaust gas andthe upstream-side NO_(x) concentration Nu which are calculated in stepS100 and step S101, and then the processing goes to step S103.

In step S103, it is determined whether there is no error informationfrom the NO_(x) sensor provided at the downstream side of the NO_(x)catalyst and the input value from the NO_(x) sensor is within a standardrange of CAN. When these conditions are not satisfied, the processing isreturned to the start position. When it is determined that both theconditions are satisfied, the processing goes to step S104.

In step S104, the NO_(x) concentration Nd at the downstream side of theNO_(x) catalyst which exists on CAN and is detected by the NO_(x) sensoris read out, the NO_(x) flow amount Nfd per unit time at the downstreamside of the NO_(x) catalyst is calculated on the basis of the NO_(x)concentration Nd at the downstream side of the NO_(x) catalyst read instep S104 and the mass flow rate Gf of the exhaust gas calculated instep S100 in step S105, and then the processing goes to step S106.

In step S106, as in the case of the step S103 described above, it isdetermined whether there is no error information from the NO_(x) sensorand the input value from the NO_(x) sensor is within the standard rangeof CAN, and also it is determined whether the system is set to anreducing agent-injection possible state (hereinafter referred to as“test environment condition TE”). When it is determined that the testenvironment condition TE is not satisfied, the system is not set to thetest-possible state, and thus the processing is returned to the startposition. On the other hand, when it is determined that the testenvironment condition TE is satisfied, the processing goes to step S107to calculate the increase amount Inc or the decrease amount Dec of theNO_(x) flow rate counter at the upstream side of the NO_(x) catalyst,and then the processing goes to step S108 (FIG. 5).

In the step S108 to which the processing is shifted when the increaseamount Inc or the decrease amount Dec of the NO_(x) flow rate Nfu at theupstream side of the NO_(x) catalyst is calculated, it is determinedwhether the temperature Tc of the NO_(x) catalyst is within a prescribedrange, the NO_(x) concentration Nu at the upstream side of the NO_(x)catalyst is within a prescribed range and the NO_(x) flow rate Nfu atthe upstream side of the NO_(x) catalyst is within a prescribed range.When all of these conditions are satisfied, the processing goes to stepS109, and the NO_(x) flow rate counter is added by the amountcorresponding to the increase amount Inc determined in step S107.Subsequently, the NO_(x) flow rate Nfu at the upstream side of theNO_(x) catalyst is integrated in step S110, the NO_(x) flow rate Nfd atthe downstream side of the NO_(x) catalyst is integrated in step S111,and they are stored in RAM.

After the NO_(x) flow rate Nfu at the upstream side of the NO_(x)catalyst and the NO_(x) flow rate Nfd at the downstream side of theNO_(x) catalyst are stored, it is determined in step S112 whether theNO_(x) flow rate counter reaches the prescribed value START and also arecording value addition flag RcrdGf is equal to zero. When all of themare satisfied, the processing goes to step S113 to read the storedintegration value of the NO_(x) flow rate Nfu at the upstream side ofthe NO_(x) catalyst, add the read-out integration value to thepreviously accumulated integration value and also store the additionresult into RAM again. Furthermore, in step S114, the stored integrationvalue of the NO_(x) flow rate Nfd at the downstream side of the NO_(x)catalyst is read, added to the previously accumulated integration valueand then stored into RAM again. Thereafter, the recording value additionflag RcrdGf is set to 1 in step S115, and then the processing goes tostep S116.

On the other hand, when it is determined in step S112 that the NO_(x)flow rate counter does not reach the prescribed value START or therecording value addition flag RcrdGf is not equal to zero, theprocessing goes to step S116, and it is determined whether the recordingvalue addition flag RcrdGf is put up. When the recording value additionflag RcrdGf is not put up, the processing is returned to the startposition. On the other hand, when the recording value addition flagRcrdGf is put up, the processing goes to step S117 to store theintegration value Nnu of the NO_(x) flow rate Nfu at the upstream sideof the NO_(x) catalyst, the integration value Nnd of the NO_(x) flowrate Nfd at the downstream side of the NO_(x) catalyst is stored in stepS118, and then the processing goes to step S119.

In step S119 to which the processing is shifted when the recording valueaddition flag RcrdGf is put up, it is determined whether the integrationvalue Nnu of the NO_(x) flow rate Nfu at the upstream side of the NO_(x)catalyst is equal to the prescribed value MIN or more. When theintegration value Nnu of the NO_(x) flow rate Nfu is less than theprescribed value MIN, the processing goes to step S120 to determinewhether the NO_(x) flow rate counter reaches the prescribed MAX. Whenthe NO_(x) flow rate counter does not reach the prescribed value MAX,the processing is directly returned to the start position. On the otherhand, when the NO_(x) flow rate counter reaches the prescribed valueMAX, the counter value is fixed to MAX in step S121, and the processingis returned to the start position.

On the other hand, when the integration value Nnu of the NO_(x) flowrate Nfu is equal to MIN or more, the processing goes to step S122 todetermine whether the NO_(x) flow rate counter reaches the prescribedvalue MAX. When the NO_(x) flow rate counter does not reach theprescribed value MAX, the processing is returned to the start positionagain. On the other hand, when the NO_(x) flow rate counter reaches theprescribed value MAX, the NO_(x) flow rate counter is fixed to theprescribed value MAX in step S123, and the processing goes to step S124.

In step S124 to which the processing is shifted when the integrationvalue Nnu of the NO_(x) flow rate Nfu is equal to MIN or more and theNO_(x) flow rate counter is fixed to MAX, it is determined whether thetimer 1 is under operation. When the timer 1 is stopped, the timer 1 isactuated in step S125, and then the processing is returned to the startposition. On the other hand, when the timer 1 is under operation, theprocessing goes to step S126.

Subsequently, in step S126, it is determined whether the timer 1 isfinished. When the timer 1 is not finished, the processing is returnedto the start position. On the other hand, when the timer 1 is finished,the NO_(x) flow rate counter is reset in step S127, and then theprocessing goes to step S135 (FIG. 6).

On the other hand, when all the conditions are not satisfied in stepS108, the processing goes to step S128 to subject the NO_(x) flow ratecounter to the subtraction processing by the amount corresponding to Decdetermined in step S107.

Subsequently, in step S129, it is determined whether the NO_(x) flowrate counter is equal to zero or not. When the NO_(x) flow rate counteris not equal to zero, the integration value is fixed in step S130, andit is determined whether the timer 1 is under operation or not in stepS131. When the timer 1 is stopped, the processing is returned to thestart position, and when the timer 1 is under operation, the processinggoes to step S126. On the other hand, when the NO_(x) flow rate counteris equal to zero, the processing goes to step S132, and it is determinedwhether the timer 1 is under operation. When the timer 1 is stopped, theintegration values Nnu and Nud are reset in step S133, and then theprocessing is returned to the start position. When the timer 1 is underoperation, the timer 1 is reset in step S134, and then the processinggoes to step S135 (FIG. 6).

In step S135, the integration value Nnu of the NO_(x) flow rate Nfu atthe upstream side of the NO_(x) catalyst is compared with theintegration value Nnd of the NO_(x) flow rate Nfd at the downstream sideof the NO_(x) catalyst to calculate the actual NO_(x) reductioncoefficient PE. Subsequently, in step S136, a threshold value PEt forthe NO_(x) purification coefficient which will be originally obtained iscalculated and set on the basis of parameter values concerning thereduction of NO_(x) under a condition under which the integration hasbeen performed, a reduction coefficient calculation completion flag isput up in step S137, and then the processing goes to step S138.

In step S138, it is determined whether the threshold value PEt of theNO_(x) reduction coefficient determined in step S136 is equal to aprescribed value PE0 or more. When PEt is smaller than the prescribedvalue PE0, the processing is returned to the start position. On theother hand, when PEt is larger than the prescribed value PE0, theprocessing goes to step S139 to determine whether the actual NO_(x)reduction coefficient PE is smaller than the threshold value determinedin step S136. When PE is smaller than the threshold value PEt, TestErroris set because the NO_(x) reduction coefficient is lowered, and thediagnosis is finished. On the other hand, when PE is larger than thethreshold value PEt, TestOK is set because the NO_(x) purificationcoefficient is kept to a predetermined value or more, and the diagnosisis finished.

According to the trouble diagnosis method for the exhaust gaspurification system based on the flowchart described above, thepurification efficiency can be calculated by integrating the NO_(x)amounts at the upstream side and downstream side of the NO_(x) catalystunder the state that the condition for normally performing the reductivereaction of NO_(x) is satisfied. Accordingly, the presence or absence ofa trouble of the exhaust gas purification system such as the abnormalityof the control system of the exhaust gas purification system, thedeterioration of the NO_(x) catalyst, the reduction in quality of thereducing agent, etc. can be accurately determined.

The construction of the exhaust gas purification system shown in FIG. 1is an example, and the exhaust gas purification system which canimplement the trouble diagnosis method of the present invention is notlimited to the thus-constructed exhaust gas purification system. Forexample, CAN may be omitted or DCU may be constructed to be integralwith the engine ECU. Furthermore, as another example, the exhaust gaspurification system may be constructed so that the circulation passageprovided for the purpose of the temperature control of the reducingagent is omitted.

1-7. (canceled)
 8. A trouble diagnosis device for an exhaust gaspurification system for passing exhaust gas discharged from an internalcombustion engine through NO_(x) catalyst to reduce NO_(x) contained inthe exhaust gas, the trouble diagnosis device comprising: upstream-sideNO_(x) flow rate calculating means for calculating the flow amount ofupstream-side NO_(x) flow amount per unit time at an upstream side ofthe NO_(x) catalyst; downstream-side NO_(x) flow rate calculating meansfor calculating a downstream-side NO_(x) flow amount per unit time at adownstream side of the NO_(x) catalyst; reduction condition determiningmeans for determining whether at least one condition for performingnormal reduction of NO_(x) is satisfied; upstream-side NO_(x) flowamount calculating means for integrating the upstream-side NO_(x) flowamount to calculate the amount of upstream-side NO_(x) passing throughthe upstream side of the NO_(x) catalyst within a predetermined timewhen a condition concerned is determined to be satisfied;downstream-side NO_(x) amount calculating means for integrating thedownstream-side NO_(x) flow amount to calculate the amount ofdownstream-side NO_(x) passing through the downstream side of the NO_(x)catalyst within a predetermined time when the condition concerned isdetermined to he satisfied; and trouble determining means for comparingthe upstream-side NO_(x) amount and the downstream-side NO_(x) amount todetermine whether the exhaust gas purification system operates normally.9. The trouble diagnosis device for the exhaust gas purification systemaccording to claim 8, wherein the upstream-side NO_(x) amountcalculating means and the downstream-side NO_(x) amount calculatingmeans store integration values which have been obtained till now whenintegration of the NO_(x) flow amount is interrupted, and resume theintegration from the stored integration values when the conditionconcerned is satisfied again.
 10. The trouble diagnosis device for theexhaust gas purification system according to claim 8, wherein theupstream-side NO_(x) amount calculating means and the downstream-sideNO_(x) amount calculating means store integration values which have beenobtained till now when integration of the NO_(x) is interrupted, andreset the integration values when the condition concerned is notsatisfied again within a predetermined time.
 11. The trouble diagnosisdevice for the exhaust gas purification system according to claim 9,wherein the upstream-side NO_(x) amount calculating means and thedownstream-side NO_(x) amount calculating means store the integrationvalues which have been obtained till now when the integration of theNO_(x) is interrupted, and reset the integration values when thecondition concerned is not satisfied again within a predetermined time.12. The trouble diagnosis device for the exhaust gas purification systemaccording to claims 9, wherein the trouble determining means finishesthe integration of the upstream-side NO_(x) flow amount and thedownstream-sidle NO_(x) flow amount when the upstream-side NO_(x) amountreaches a predetermined value, and compares the upstream-side NO_(x)amount and the downstream-side NO_(x) amount.
 13. The trouble diagnosisdevice for the exhaust gas purification system according to claims 10,wherein the trouble determining means finishes the integration of theupstream-side NO_(x) flow amount and the downstream-side NO_(x) flowamount when the upstream-side NO_(x) amount reaches a predeterminedvalue, and compares the upstream-side NO_(x) amount and thedownstream-side NO_(x) amount.
 14. The trouble diagnosis device for theexhaust gas purification system according to claim 8, wherein thetrouble determining means compares the ratio between the upstream-sideNO_(x) amount and the downstream-side NO_(x) amount with a thresholdvalue which is determined in accordance with a condition under whichreduction of NO_(x) is normally performed for determination.
 15. Thetrouble diagnosis device for the exhaust gas purification systemaccording to claim 9, wherein the trouble determining means compares theratio between the upstream-side NO_(x) amount and the downstream-sideNO_(x) amount with a threshold value which is determined in accordancewith a condition under which reduction of NO_(x) is normally performedfor determination.
 16. The trouble diagnosis device for the exhaust gaspurification system according to claim 10, wherein the troubledetermining means compares the ratio between the upstream-side NO_(x)amount and the downstream-side NO_(x) amount with a threshold valuewhich is determined in accordance with a condition under which reductionof NO_(x) is normally performed for determination.
 17. The troublediagnosis device for the exhaust gas purification system according toclaim 8, wherein the upstream-side NO_(x) flow rate calculating meansperforms its calculation on a basis of an NO_(x) concentrationdischarged from the internal combustion engine which is calculated froman operation state of the internal combustion engine.
 18. The troublediagnosis device for the exhaust gas purification system according toclaim 9, wherein the upstream-side NO_(x) flow rate calculating meansperforms its calculation on a basis of an NO_(x) concentrationdischarged from the internal combustion engine which is calculated froman operation state of the internal combustion engine.
 19. The troublediagnosis device for the exhaust gas purification system according toclaim 10, wherein the upstream-side NO_(x) flow rate calculating meansperforms its calculation on a basis of an NO_(x) concentrationdischarged from the internal combustion engine which is calculated froman operation state of the internal combustion engine.
 20. A troublediagnosis method for an exhaust gas purification system for diagnosingthe presence or absence of a trouble of an exhaust gas purificationsystem in which exhaust gas discharged from an internal combustionengine is passed through NO_(x) catalyst to reduce NO_(x) contained inthe exhaust gas, the method comprising: calculating all upstream-sideNO_(x) flow amount and a downstream-side NO_(x) flow amount per unittime at upstream and downstream sides of the NO_(x) catalyst;determining whether at least one condition under which the reduction ofNO_(x) is normally performed is satisfied or not; integrating theupstream-side NO_(x) flow amount and the downstream-side NO_(x) flowamount when a condition concerned is satisfied, and calculating anupstream-side NO_(x) amount and the downstream-side NO_(x) amountpassing through the upstream side and the downstream side of the NO_(x)catalyst within a predetermined time; and comparing the upstream-sideNO_(x) amount with the downstream-side NO_(x) amount to determinewhether the exhaust gas purification system operates normally.